Purpose :
To estimate the resolution in estimates of nerve fiber bundle loss measured as a fraction of PIMD integrated over the optic nerve head (ONH), PIMD-[Low angular limit;Upper angular limit].

Methods :
3D represenations of the ONH were captured with Topcon OCT-2000 from one eye of each of totally 7 subjects with early stage open angle glaucoma. Altogether, 3 volumes were captured at two occasions within 3 months and PIMD was semi-automatically segmented in each volume in 500 equally spaced angular segments distributed over 2Pi in the frontal plane, around the manually identified center of the ONH. Each angular segmentation of PIMD-2Pi started at approximately 3 o’ clock in the frontal plane and PIMD was sequentially estimated anticlock-wise. In post processing, the rotation of PIMD-2Pi was adjusted in the frontal plane within segmentations, volumes and occasions so that each angular fraction of each recording was the same for all recordings. In each segmented PIMD-2Pi, the individual PIMDs were suborganized into sequential bundles of two adjacent PIMDs. Then, the estimated PIMDs were analyzed with an analysis of variance to estimate the variance for subjects, occasions, volumes, segmentations, bundles and adjacent PIMDs.

Results :
The estimated variances were for subjects, 923 x10-4 mm, for occasions negligible in relation to volumes, for volumes 1.79 x10-4 mm, for segmentations 0.256 x10-4 mm, for bundles 24 x10-4 mm and for adjacent PIMDs 0.0489 x10-4 mm. A typical early glaucomatous nerve fiber bundle notch in the ONH subtends one clock hour. Considering 500 PIMDs per full ONH, approx. 40 PIMDS subtend one nerve fiber notch. The estimated variances imply that recording 3 volumes per visit every 3rd month a statistically significant change of a 1 clock hour notch at alpha = 0.05 can be detected with a power of 0.8 already between the 1st and 2nd follow-up visit.

Conclusions :
Angular correction in the frontal plane of recordings of angular PIMD of the ONH allows comparison of angular segments of PIMD-2Pi. This substantially increases the resolution of PIMD for reduction of PIMD within a specific angular segment.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.